CAPTCHA, standing for “Completely Automated Public Turing test to tell Computers and Humans Apart”, is an automatic challenge-response test to distinguish between humans and machines. Over the past decade, text-based CAPTCHA (TBC) have become popular in preventing adversarial attacks and spam in many websites and applications including emails services, social platforms, web-based market places, and recommendation systems. However, in addition to several problems with TBC, it has become increasingly difficult to solve in recent years, to keep up with OCR technologies. Thus, there may be a need for efficient CAPTCHA techniques.